GER-2681B
MULTILIN
GE Power Management
Phase
Comparison
Relaying
PHASE COMPARISON RELAYING
INTRODUCTION
Phase comparison relaying is a kind of differential relaying that compares the phase angles of the
currents entering one terminal of a transmission line with the phase angles of the currents entering
all the remote terminals of the same line. For the conditions of a fault within the protected zone
(internal fault), the currents entering all the terminals will be in phase. For conditions of a fault
outside the zone of protection (external or through fault), or for just plain load flow, the currents
entering any one terminal will be 180 degrees out of phase with the currents entering at least one of
the remote terminals. The phase comparison relay scheme makes this phase angle comparison and
trips the associated breakers for internal faults. Since the terminals of a transmission line are
normally many miles apart, some sort of communication channel between the terminals is required
to make this comparison.
phase basis. That is, it would not be possible to
compare all three phase currents at terminal A
individually with all three at terminal B over one
single channel and one single comparing unit.
Thus, in the interest of economy, all three phasecurrents are mixed to produce a single phase
quantity whose magnitude and phase angle
have a definite relation to the magnitude and
phase angle of the three original currents. It is
this single phase quantity that is phase
compared with a similarly obtained quantity at
the remote end(s) of the line.
FUNDAMENTAL PRINCIPLE OF PHASE
COMPARISON
The basic operation of a phase comparison
scheme requires that the phase angle of two or
more currents be compared with each other. In
the case of transmission line protection, these
currents may originate many miles from each
other so, as noted above, some form of
communication channel is required as part of
the scheme. If a two-terminal line is considered,
the relays located at terminal A can measure the
current at that terminal directly. The phase angle
of the current at the remote terminal (B) must
somehow be communicated to terminal A.
Since the current sine wave is positive for one
half cycle and then negative for the next half
cycle etc., it may be used to key a transmitter
first to a MARK signal for a half cycle and then
to a SPACE signal for the next half cycle for as
long as the current is present. Such a signal
transmitted at B and received at A can be
compared with the current at A to determine
whether the two quantities are in phase or out
of phase with each other. Conversely, the current
at terminal B may be compared with the signals
received from terminal A.
While there are many variations on the basic
scheme (and these will be discussed
subsequently), the general method employed to
compare the phase angle, or phase position of the
currents is always the same. The left side of Figure
1 illustrates the conditions for a fault internal to
the protected zone. The top sketch shows about
1 cycle of the current flowing into terminal A. The
second sketch down indicates the current flowing
into terminal B. These are in phase since the fault
is internal. The third sketch down represents the
receiver output at terminal A as a result of the
transmitter at B being keyed from a signal
produced by the current at that terminal.
It becomes apparent that a comparison such as
that described above must be made on a single
The MARK-SPACE designations given to the
received signal are for identification and have no
1
special significance. If the communication
equipment happened to be a simple radio
frequency transmitter-receiver, and if the
positive half cycle of current keyed the
transmitter to ON, then the MARK block would
correspond to a received remote signal while the
SPACE block would correspond to no signal.
Conversely, if the negative portion of the current
wave keyed the transmitter to ON, then the
SPACE block would represent the received
signal.
If the communication equipment happened to
be a frequency shift channel so that both the
MARK and the SPACE signals were definite
outputs, Figure 2A would represent a tripping
scheme since tripping is predicated on the
receipt of a remote MARK or tripping signal. On
the other hand, Figure 2B would represent a
blocking scheme in as much as it will block
tripping in the presence of a SPACE or blocking
signal. It will trip only in the absence of this
signal.
With a frequency-shift transmitter-receiver as the
communication equipment, the MARK block
would represent the receipt of the hi-shift
frequency and the SPACE block the lo-shift
frequency if the remote transmitter were keyed to
high from a positive current signal. The converse
would be true if the transmitter were keyed to high
from a negative current signal. In any case the
MARK block received at A, whatever it represents,
corresponds to positive current at B while the
SPACE block corresponds to negative current at B.
The right side of Figure 1 illustrates the
conditions during an external fault. Referring to
Figures 2A and 2B it will be noted that neither
approach, the blocking or the tripping, will
result in a trip output for this condition since the
AND circuits will never produce any outputs to
the 8.33/18 timers.
At this point it should be explained that the
conditions illustrated in Figure 1 are ideal. They
seldom, if ever, occur in a real power system.
Actually an internal fault would not produce a
received signal MARK-SPACE relationship that
is exactly in phase with the locally contrived
single phase current. This is true for a variety of
reasons including the following:
If we consider an internal fault as shown on the left
side of Figure 1, the relay at A would be comparing
quantities illustrated in the top and third-from-thetop sketches. If these two signals at terminal A
were to be compared as shown in Figure 2A over a
frequency-shift equipment, a trip output would
occur if positive current and a receiver MARK
signal were both concurrently and continuously
present for at least 8.33 milliseconds. The trip
output would be continued for 18 milliseconds to
ride over the following half cycle during which the
current is negative, and the half cycle after that
when the pick-up timing takes place again.
(a) Current transformer saturation.
(b) Adjustment differences in the current mixing
networks in the relays at both ends of the
line.
(c) Phase angle differences between the currents
entering both ends of the line as a result of
phase angle differences in the driving system
voltages.
Assuming that the MARK and SPACE signals
cannot both be present concurrently then it might
be argued that a comparison could be made
between the positive half cycle of current and the
absence of a receiver SPACE output, Figure 2B
illustrates this logic.
(d) Transit time of the communication signal.
(e) Unsymmetrical build-up and tail-off times of
the receiver.
2
3
4
Thus, the logic shown in Figures 2A and 2B
would rarely, if ever, produce a trip output on an
internal fault because the 8.33 milliseconds
(which is the time of a half cycle on a 60 hertz
base) requires perfect matching. In actual
practice a 3-4 millisecond setting is used rather
than the 8.33 setting illustrated. This makes it
much easier to trip on internal faults. It also
makes it much easier to trip undesirably on
external faults. However, experience has
indicated that with proper settings and
adjustments in the relay such a timer setting
offers an excellent compromise. This may be
better appreciated if it is recognized that item
(a) above is generally minimized and item (c) is
nonexistent on external faults.
were done, in Figure 2A for example, it would
have been necessary to compare the presence of
negative current with a received SPACE signal
rather than a MARK signal.
It should be recognized that the above
discussion, as well as Figures 2 and 3, are
rudimentary. The complete phase comparison
scheme is considerably more sophisticated and
will be discussed in more detail subsequently.
However, at this point it would be well to note
that phase comparison on a continuous basis is
not permitted mainly because it would tend to
reduce the security of the scheme. For this
reason fault detectors are provided. They initiate
phase comparison only when a fault occurs on,
or in the general vicinity of, the protected line. A
simplified sketch of the logic of a phase
comparison blocking scheme including fault
detectors is illustrated in Figure 4. This is a
somewhat more fully developed version of
Figure 2D, and the same logic is present at both
ends of a two-terminal line.
In the event that an ON-OFF communication
equipment were to be employed rather than the
frequency-shift equipment, the logic would appear
as in Figures 2C and 2D. It will be noted in these
two figures that the reference to MARK and SPACE
have been conveniently omitted since the receiver
output is either present or not as against the case
of the frequency-shift equipment where it could be
there in either of two states. Figure 2C illustrates a
tripping scheme while Figure 2D a blocking
scheme. Here again, the 8.33 millisecond timer is,
in practice, actually set for 3-4 milliseconds.
It will be noted from Figure 4 that AND1 (the
comparer) at each end of the line compares the
coincidence time of the positive half cycle of
current with the absence of receiver output. This
is initiated only when a fault is present as
indicated by an output from FDH. FDH is set so
that it does not pick up on load current but does
pick up for all faults on the protected line
section. Thus, when a fault occurs FDH picks up,
and if the receiver output is not present for
3 milliseconds during the positive half cycle of
current out of the mixing network, a trip output
will be obtained.
Figures 3A, 3B, 3C, and 3D are for three-terminal
lines and they correspond directly to Figures 2A,
2B, 2C, and 2D. It will be noted from Figure 3 that
for a three-terminal line, the relay at A must
receive information from both the remote
terminals. The same applies to the relays at
terminals B and C. As in the case of the twoterminal lines, the 8.33 millisecond timer
illustrated in Figure 3 will actually be set for 3-4
milliseconds.
Of course, the output from the receiver will
depend on the keying of the remote transmitter.
The transmitters at all line terminals are keyed
in the same manner. They are keyed ON by an
output from FDL and keyed OFF by the squaring
amplifier via AND2 during the positive half
cycles of current. The FDL function is required at
While all the sketches in Figures 2 and 3
compare the positive half cycle of current with a
receiver output, the negative half cycle might
just as well have been selected. However, if this
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all terminals in all phase comparison blocking
schemes to initiate a blocking signal from the
associated transmitter. This is received at the remote
receiver and blocks tripping via the comparer during
external faults. FDL has a more sensitive setting and
therefore operates faster than the remote FDH
function. It is obvious from Figure 4 that if an
external fault occurred, and FDL did not operate at
least as fast as the remote FDH, false tripping could
occur because of the lack of receiver output. In
general FDL is set so as not to pick up on load
current but still with a lower pick up than FDH so
that it will operate before FDH. For an internal fault,
the currents entering both ends of the line are in
phase with each other. Thus, during the half cycle
that the SQ AMP is providing an input to AND1, the
associated receiver is producing no output, and so
tripping will take place at both ends of the line. For
an external fault, the current entering one terminal is
180 degrees out of phase with the current entering
the other terminal. Under these conditions, during
the half cycles when the SQ AMP is producing
outputs, the associated receiver is also providing an
output thus preventing an AND1 output. No tripping
will take place.
PHASE COMPARISON EXCITATION
Before discussing this subject it is well to
consider what takes place in terms of the
currents that are available for comparison when
a fault occurs on a power system. Table I below
lists the sequence components of fault current
that are present during the various different
kinds of faults while Figure 5 illustrates the
relative phase positions of the sequence
components of fault current for the different
kinds of faults and the different phases involved.
TABLE I
Sequence Components
Type of Fault
Positive Negative Zero
Single-Phase-to-Ground
yes
yes
yes
Phase-to-Phase
yes
yes
no
Double-Phase-to-Ground
yes
yes
yes
Three-Phase
yes
no
no
Figure 5 shows the relative phase positions of
the outputs of a positive sequence network, a
negative sequence network, and a zero
sequence network all referenced to phase A. The
transfer functions of these three networks are
given by the following equations.
VARIATIONS IN PHASE COMPARISON
SCHEMES
1
3
1
I2 =
3
1
I0 =
3
I1 =
There are a number of different phase comparison
schemes in general use today and while all of these
employ the same basic means of comparison
described above, significant differences do exist.
These differences relate to the following:
(Ia + Ib /120° + Ic /-120°)
(1)
(Ia + Ib /-120° + Ic /120°)
(2)
(Ia + Ib + Ic)
(3)
It is interesting to note that the phase positions
of the sequence network outputs differ
depending on the phase or phases that are
faulted as well as the type of fault. For example,
while the positive, negative, and zero sequence
components are all in phase for a single-phaseA-to-ground fault, they are 120 degrees out of
phase with each other for phase-B-to-ground,
and phase-C-to-ground faults.
(1) Phase comparison excitation (component or
current to be compared).
(2) Pure phase comparison vs. combined phase
and directional comparison.
(3) Blocking vs. tripping schemes.
(4) Single vs. dual phase comparison.
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pure positive sequence phase comparison
appears practical only in a minority of the cases
and so is not suitable for a scheme that is to be
generally applicable.
It will be observed from Table I that positive
sequence currents are available for all kinds of
faults, negative sequence currents are available
for all but three-phase faults, and zero sequence
currents are available only for faults involving
ground. Thus, it appears that if one single
sequence component of current were to be
selected for use to make the phase comparison,
the positive sequence component would suffice.
Actually this is not the case in many if not most
of the applications because of the presence of
through load current during the fault.
Significant negative sequence currents are
present only during faults, they are present in all
but balanced three phase faults, and there is no
significant negative sequence component of
load current. All this combines to make pure
negative sequence ideal for phase comparison
except that it will not operate for balanced three
phase faults. Similar comments may be made
regarding pure zero sequence phase comparison
with the additional limitation that it will not
operate for phase-to-phase faults. Thus, there
does not appear to be one single sequence
component or one single phase current that
could be used in a phase comparison scheme to
protect against all types of faults.
For a single-phase-to-ground fault on the
protected line, the positive sequence
component of fault current entering one end will
be in phase with that entering the other end.
This is a tripping situation for the phase
comparison scheme. However, any load flow
across the line during the fault will produce a
positive sequence component of load current
entering one end of the line that is 180 degrees
out of phase with that entering the other end.
(That is, the positive sequence component of
load current entering one end is in phase with
that leaving the other end). This is a nontripping situation for the phase comparison
scheme. The phase position of the load
component relative to the fault component
depends on such factors as the direction of the
load flow, power factor of the load flow, and the
phase angles of the system impedances. The
phase position of the "net" (load plus fault)
positive sequence current entering one end of
the line relative to that entering the other end
will depend on these same factors plus the
relative magnitude of the fault and load
components of current.
There are a number of different approaches that
are possible to provide a complete scheme.
Probably the most obvious would be to make
the phase comparison on each phase separately.
This is undesirable principally because the cost
would be high since three separate phase
comparison relays and communication sets
would be required. Another approach would be
to use two separate phase comparison relays
and communication sets, one for pure positive
and the other for pure negative sequence
currents. The latter would serve to protect
against all unbalanced faults while the former
would take care of three phase faults and also
provide a measure of back-up protection for
heavy unbalanced faults. Here again cost is an
important factor.
As soon as consideration is given to the use of a
separate positive and a separate negative phase
sequence comparison, the idea of switching
from one to the other presents itself. Such
schemes are available. They include detectors
separate from the phase comparison function
In general, the heavier the fault current, and the
lighter the load current, the more suitable is the
use of pure positive sequence for phase
comparison. Heavier line loadings and lower
fault currents will tend to make the scheme less
apt to function properly for internal faults. Thus,
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(1) Which sequence components should be
mixed with the positive sequence.
that distinguish between three phase faults and
all other types. For three phase faults the
negative sequence network is unbalanced so
that it produces an output for positive sequence
current as well as for negative sequence current.
The scheme operates normally to provide
negative sequence phase comparison for all
unbalanced faults. When a three phase fault
occurs the three-phase detectors at both ends of
the line operate to automatically unbalance
their respective negative sequence networks
and make them sensitive to positive as well as
negative sequence currents. Since the fault is
three phase, there is no negative sequence
current produced so the phase comparison
is made on a pure positive sequence basis.
This is all accomplished with a common
communication channel for both modes.
(2) What percentages of the full magnitude of
each sequence component of current should
be used.
Figure 6 illustrates a two-terminal line with an
internal phase B-to-ground fault. The phasor
diagrams at the top of the page indicate the
phase positions of the sequence currents at
both ends-of the line assuming the positive
direction of current flow into the line, and also
assuming phase A reference as in equations (1),
(2), and (3) previously given.
At this point it should be recognized that the
positive sequence component of current is
made up of two parts, the load component (IL1)
and the fault component (IF1). By an analysis
utilizing superposition the load component (IL1)
may be established as the current flowing just
prior to the fault. The three fault components of
current (IF1, I2 and l0) are then calculated using
the voltage that existed at the point of fault just
prior to the fault. Since the load component of
current is equal to the vector difference between
Bus X and Bus Y voltages divided by the
impedance of the line, and since the prefault
voltage (at the point of fault) has a phase
position somewhere between that of X and Y
voltages, the positive sequence component of
fault current will be displaced from the load
component by about 90 degrees plus or minus
about 30 degrees. The phasor diagrams at the
top of Figure 6 assume that load current flow is
from bus X to bus Y. The first row of the table in
Figure 6 indicates that for the conditions
assumed, the net positive sequence current
entering both ends of the line are about 120
degrees displaced from each other. Heavier fault
current and lighter load current would reduce
this angle toward zero while the converse would
increase the angle toward 180 degrees.
Another similar approach would be to provide
two separate sequence networks, one pure
positive sequence and the other pure negative
sequence. Then use the three-phase detector to
switch the logic so that only for three phase
faults the outputs of the positive sequence
networks at both ends of the line are compared
but for all other faults the negative sequence
outputs are compared. Here again all this being
accomplished over a common channel. This
approach has never been used possibly because
of the idea of using "Mixed Excitation."
Mixed Excitation is a term used to describe a
phase comparison scheme that mixes the
outputs of the different sequence networks in a
given proportion and phase angle and then
makes a phase comparison for all faults based
on this mix. Thus, all such schemes must
include positive sequence plus negative
sequence and/or zero sequence in order to
operate for all faults. The two main questions to
be resolved are:
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10
(c) The effects of load current must be
minimized. Thus, negative and/or zero
sequence components should be weighted
over the positive sequence components.
The second and third rows of the table of Figure 6
indicates the relative phase positions of the
positive plus negative, and positive plus negative
plus zero sequence components respectively.
These appear to be more unsatisfactory. Rows 4
and 5 combine the components differently and
both appear to yield much better results.
(d) The limits of application should be broad
enough to render the scheme useful as a
protection tool.
It is obvious, from Figure 5 that a similar fault on
a different phase would yield different results.
This is illustrated in Figure 7 where a phase-Ato-ground fault at the same location is analyzed.
As noted earlier, the integrator timers in phase
comparison schemes are generally set for about
3 milliseconds. This will permit tripping on
internal faults with as much as 115 degrees
between the phase angles of the currents
entering both ends of the lines. On this basis, only
excitation by I2 - I1/5 would prove satisfactory for
the two cases studied in Figures 6 and 7.
In line with the considerations stipulated above,
the best overall results using mixed excitation
would be attained by using I2 - I1/K, where K is a
constant that is adjustable within limits. While it
is likely that the inclusion of zero sequence
excitation would be helpful for one case or
another, it is not generally employed because
the problem of evaluating the overall
performance of the scheme would be magnified
considerably. This is true mainly because the
current distribution in the zero sequence
network is generally quite different from that in
the positive and negative sequence networks
where
the
current
distributions
are
approximately the same. For any given fault on
a transmission line, the ratio of I F1/I2 at any
terminal is the same as at any other terminal of
that line. This is not true of either IF1/I0 or I2/l0. It
is this that makes the use of zero sequence
excitation undesirable.
Actually only two simple faults were
investigated. It is obvious that different results
would have been obtained for these same kind
of faults if the relative magnitudes of load
current, positive sequence fault current, and zero
sequence fault current had been assumed
differently. Also, for the values of currents
assumed, different results would obtain for
other types of faults. In addition, if different
combinations and weighting factors of the
sequence components had been investigated
still different answers would have resulted. In
the proper selection of sequence components
and weighting factors for Mixed Excitation
phase comparison the following points must be
considered:
Mixed Excitation Phase Comparison
If the mixing network of Figure 4 were designed
to produce an output that is proportional to
I2 – I1/K, this logic would then be a simplified
representation of a mixed excitation phase
comparison scheme. In such schemes, the pick
up setting of FDH must be high enough so that
the I1/K output from the mixing network does
not result in continuous phase comparison on
load current (I2 is normally zero during normal
system conditions). Also, it may be desirable to
have FDL set to pick-up at some level above full
load so that channel is not keyed on and off
continuously during normal load conditions.
(a) Whatever combination and weighting
factors are employed, the application rules
should be simple enough to make the
application practical.
(b) As a corollary to (a) above, the fewest number
of sequence components should be used.
11
Since FDH is set higher than FDL, this
requirement results in a still higher setting for
FDH.
the line and is set to reach beyond the remote
terminal(s). The MB function "looks" backwards,
out of the line and is set to reach beyond the
reach of the remote MT function(s). These
functions are required to operate for three phase
faults but will incidentally also operate for some
other faults involving one or both of the
associated phases.
Because FDH controls tripping, this arrangement
limits the applicability of the basic scheme to
circuits where the minimum three phase fault
current is significantly higher than the maximum
load current.
The basic idea is to compare the relative phase
positions of the mixed excitation (I2 - I1/K) in the
normal manner but to initiate the comparison
with more discriminatory fault detectors. To
accomplish these ends, pure negative sequence
is used to operate FDH and FDL. Since there is
no significant negative sequence current
flowing during normal system conditions, these
fault detectors may be applied with very
sensitive settings. They will initiate phase
comparison for all except three-phase faults. In
the event of three phase faults the MT and MB
units function as FDH and FDL respectively.
Since these functions can discriminate between
load currents and fault currents regardless of
magnitudes, they can detect faults that produce
currents less than full load values. Thus, the
combination of negative sequence current level
detectors and distance measuring functions
provide a means for more sensitive fault
detection.
The requirements for the satisfactory
performance of a mixed excitation scheme using
overcurrent fault detectors (FDH and FDL) are
noted below.
(a) Both the FDL and FDH fault detectors must
be set above full load current.
(b) All internal faults regardless of type or the
particular phases involved must produce
enough I2 - I1/K to operate FDH at all ends of
the line.
(c) FDL must be set with a lower pick-up than
FDH at the remote end(s) of the line for
security during external faults.
(d) The phase angle difference between the
I2 - I1/K quantities obtained at all terminals of
the protected line during all types of internal
faults, and for any combination of phases,
must be less than 115 degrees.
With this arrangement, the requirements for
satisfactory performance are given below.
Mho Supervised Mixed Excitation Phase
Comparison
(a) The FDH function must be set sensitively
enough to pick up for all unbalanced faults
on the protected line section.
Figure 8 is an abbreviated logic diagram
covering a modified mixed excitation blocking
scheme that provides somewhat more
sensitivity than the basic scheme. In order to
accomplish this, two single-phase directional
mho distance measuring functions are used.
These are both associated with the same pair of
phases which, in this case, are phases A and B.
The MT function at each terminal "looks" into
(b) FDL must be set with a lower pick up than
FDH at the remote end(s) of the line for
security during external faults.
(c) The MT function must be set with a long
enough reach to detect all internal three
phase faults.
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13
14
(d) The MB function must be set to outreach all
remote MT units.
With this kind of scheme, the requirements for
satisfactory performance are given below:
(e) The phase angle difference between the
I2 - I1/K quantities obtained at all terminals of
the protected line during all types of internal
faults, and for any combination of phases
must be less than 115 degrees.
(a) The FD function must be set with a pick up
that is above full load current.
(b) The FDH function must be set to pick up,
with the network unbalanced, at some level
of positive sequence current that is higher
than the FD pick up setting
Network Unbalancing Phase Comparison
(c) The FDL function must be set with a pick up
below that of the remote FDH unit in order to
maintain security during external faults.
Earlier in this section reference was made to
schemes that employ pure negative sequence
phase comparison except for the case of a three
phase fault for which the sequence network is
unbalanced to produce sensitivity to positive
sequence currents. Figure 9 illustrates, in an
abbreviated manner, the logic of such a blocking
type of a scheme. It will be noted that unless all
three of the FD units pick up, the phase
comparison will be on a negative sequence
basis. When all three FD units pick up, as they
will for three phase faults, the network is
unbalanced to produce an output for balanced
positive sequence current inputs, and phase
comparison takes place on a pure positive
sequence basis. The FD units must be set to pick
up above maximum full load current in order to
insure pure negative sequence comparison on
all but three-phase faults.
(d) FD must pick up for all internal three phase
faults.
(e) FDH must pick up for all internal faults.
Separate Positive & Negative Sequence Phase
Comparison
The scheme described earlier in this section that is
compromised of two separate phase comparison
schemes may be represented in abbreviated logic
by two diagrams similar to Figure 4. In one scheme
the mixing network would be a pure negative
sequence network. In the other scheme it would
be a pure positive sequence network. In order for
this approach to perform satisfactorily, the
following requirements must be met.
Since phase comparison is initiated by the same
FDH for all kinds of faults, the ratio of positive
sequence pick up to negative sequence pick up
depends only on the amount of unbalance in the
network introduced by FD operation. This is
usually adjustable in the relay. In this kind of
scheme FDH is set so that it picks up at a value
of positive sequence current that is somewhat
higher than the three phase fault current
required to operate all three FD units. The
response of FDH to negative sequence currents
is then dependent on the network unbalance
described above. FDL is set with a pick up that
is below that of FDH.
(a) The negative sequence FDH must be set so
that it operates for all unbalanced faults
internal to the protected section.
(b) The negative sequence FDL must be set
somewhat more sensitively than the remote
FDH for security during external faults.
(c) The pick up of the positive sequence FDL
must be set above full load to prevent
continuous transmission under load.
15
2. Phase comparison relays with overcurrent
fault detectors do not require a system
potential supply in order to operate.
(d) The pick up of the positive sequence FDH
must be set somewhat higher than that of
FDL at the remote end of the line for security
during external faults.
3. Phase comparison schemes are generally
suitable for use on series compensated lines
where distance type schemes may not be
suitable.
(e) The positive sequence FDH must pick up for
all three phase faults internal to the
protected line section.
4. In their simple form, phase comparison
schemes are relatively inexpensive.
It should be recognized that this scheme will not
detect three phase faults unless the fault current
exceeds full load currents.
5. Except for the mho supervised schemes
phase comparison relays will not operate
during system swings and out-of-step
conditions.
Summary of Phase Comparison Excitation
Considerations
The following is a brief summation of the
foregoing discussions.
COMBINING PHASE AND DIRECTIONAL
COMPARISON
1. There are a number of different ways in which
phase comparison relaying may be arranged.
However, in every case some combination or
arrangement of positive and negative
sequence components of current offers the
best general approach.
Since there is no single sequence component
that could be used in phase comparison
schemes to provide protection for all types of
faults it is necessary to compensate for this
deficiency. The previous section discusses
means for mixing sequence components,
unbalancing sequence networks for certain
faults, and using two or more complete
schemes each with different excitation. Another
approach that is possible and that has gained
acceptance is called "Combined Phase And
Directional Comparison."
2. Mixed excitation schemes are generally more
difficult to apply than are schemes that
compare pure sequence components.
3. Unless some sort of distance type fault
detectors are applied, none of these schemes
can detect three phase faults that produce
currents that are below full load values.
As the name implies, a combined phase and
directional comparison scheme combines the
principles of both phase comparison and
directional comparison in one single scheme
utilizing a common communication channel.
The basic approach is to use pure negative or
pure zero sequence for the phase comparison
plus a set of mho functions at each end of the
line for the directional comparison portion. One
mho function (MT) operates for faults in the
tripping direction. The other (MB) operates for
faults in the blocking direction. Figure 10
In light of the above, the question of, "Why
Phase Comparison?" might come to mind.
Some of the answers are given below.
1. Phase comparison relaying is not affected by
zero sequence mutual impedances that can
cause directional type ground relays to
misoperate.
16
17
detectors, and squaring amplifier would all
operate for some faults, it is necessary to set up
an order of preference between the two modes
of operation. For reasons that will be discussed
subsequently, these combined schemes give
preference to the directional comparison (mho
functions) over the phase comparison (fault
detectors), as may be observed from Figure 10.
If an internal fault were to occur for which both
modes functioned, the MT functions at both
ends of the line would prevent any signal
transmission by blocking AND2. This is true
regardless of the attempt of FDL to start
transmission. Also, the lower input to AND1
would be made continuously present by MT
regardless of FDH and the squaring amplifier.
illustrates the rudiments of a combined phase
and directional comparison blocking scheme.
Such a scheme is in general more sensitive than
the schemes described earlier. This is so
because the phase comparison, since it does not
include positive sequence excitation, may be set
to operate for low values of fault current. On the
other hand the distance relays are basically able
to discriminate between fault currents and load
currents. Thus, the overall scheme will operate
for fault currents well below full load current.
Referring to Figure 10 and assuming that the MT
and MB functions do not exist, it will be observed
that the scheme becomes the same as the simple
phase comparison scheme of Figure 4. Thus, for
any fault internal or external to the protected
section for which no mho unit operates, the
scheme will perform as a simple phase
comparison scheme. On the other hand if it is
assumed for the moment that the fault
detectors and the squaring amplifier are
inoperative, the scheme behaves as a simple
directional comparison scheme under control of
MT and MB. For an internal fault the MT units at
both ends of the line operate to stop all
transmission via AND2. At the same time they
provide the lower input to AND1. With no
receiver outputs, AND1 provides a signal to the
associated integrator which after 3 milliseconds
produces a trip output. For an external fault, say
to the left of circuit breaker A, MB at A would
operate to key the transmitter on. This results in
continuous transmission of a blocking signal to
the receiver at B. The output of the receiver at B
blocks any MT operation at B from producing an
AND1 output at that terminal. This in turn
prevents any trip output from the integrator. At
terminal A, since the fault is in the blocking
direction, MT does not operate, the lower input
to AND1 is not present, and no tripping can take
place.
During an external fault to the left of terminal A
for example, MB at that terminal will operate to
key on the transmitter to send a continuous
blocking signal to the remote end. At the same
time MB will block AND4 from permitting the
squaring amplifier from stopping carrier every
other half cycle. This in turn will block tripping
at terminal B. MB also blocks AND3 from
allowing FDH to provide a comparer input. Thus,
terminal A will not trip either.
The need for this directional comparison
preference will become apparent if an external
fault is considered to be just to the right of
terminal B. Assume that for this fault both the
phase comparison and the directional
comparison detectors would operate. Thus, at
terminal B the MB function will key its
transmitter to send a continuous blocking
signal. At terminal A the MT function will see
the fault and attempt to trip but will be blocked
by a continuously received blocking signal from
the receiver. If the phase comparison squaring
amplifier at B were permitted to key the
transmitter off every other half cycle, the MT
function at A would cause a trip during that half
cycle. This is so because AND1 would produce
alternate half cycle output pulses that would in
Since in the combined scheme it is possible, and
even likely, that the mho functions, fault
18
turn time out the integrator. For this reason, and
because it is likely that some external faults will
result in the operation of both modes,
directional comparison preference is employed.
(c) The MT function must be set with a reach
that is long enough to enable it to see all
multi-phase faults on the protected line.
(d) The MB function must be set to reach further
than the remote MT unit for security during
external faults.
Zero Sequence Excitation
As noted earlier, in combined phase and directional
comparison schemes, the excitation could be pure
zero or pure negative sequence current.
Considering pure zero sequence excitation and
referring to Figure 10, the sequence network would
be a zero sequence network. Actually no network
would be required in the relay for this case because
the wye connected CT's normally used are in fact a
zero sequence network in themselves. With zero
sequence excitation the phase comparison portion
of the overall scheme would not be capable of
operating for phase-to-phase and three-phase
faults. For this reason the directional comparison
portion must include MT and MB functions that
can detect and operate for faults involving any two
or more phases. This requires that the MT and MB
functions each be three single phase mho units.
(e) The MB function must not operate for any
internal single-phase-to-ground fault for
which the associated MT function does not
operate otherwise tripping will be blocked.
Negative Sequence Excitation
It should be noted that distance relays designed to
operate for faults involving two or more phases
will operate for double-phase-to-ground faults and
also for certain close-in single-phase-to-ground
faults. Thus, it is reasonable to expect that both
the phase and directional comparison modes will
be activated for many faults and so the preference
is required.
The logic shown in Figure 10 will apply as well
to negative sequence phase comparison
excitation as it does to zero sequence excitation.
In this case, however, the sequence network
illustrated would have to be a negative
sequence rather than a zero sequence network.
Also, since the negative sequence phase
comparison will protect against all unbalanced
faults, the MT and MB (directional comparison)
functions are required only for three-phase fault
protection. However, if these functions are
designed to respond to all multi phase faults,
then phase-to-phase and double phase-toground faults will be protected by both modes
while single-phase-to-ground faults will be
protected by only the phase comparison mode,
and three phase faults by only the directional
comparison mode.
In order to obtain satisfactory performance from
such a scheme, the following requirements must
be met:
In order to obtain satisfactory performance from
such a scheme, the following requirements
must be met.
(a) The FDH function must be set so that it
operates for all single-phase-to-ground faults
internal to the protected line.
(a) The FDH function must be set so that it
operates for all unbalanced faults internal to
the protected line.
(b) The FDL function must be set somewhat
more sensitively than the remote FDH for
security during external faults.
(b) The FDL function must be set somewhat
more sensitively than the remote FDH for
security during external faults.
19
6. In general both schemes will have the same
sensitivity. That is, the 3I0 sensitivity of the
fault detectors in the zero sequence scheme
will be about the same as the I2 sensitivity of
the fault detectors in the negative sequence
scheme. For some long lines with strong
ground sources, the distribution of fault
currents may be such that for faults near one
terminal the I2 at the remote terminal is
greater than 3I0 at the same terminal. For
such applications the negative sequence
scheme may be best.
(c) The MB function must be set to reach further
than the remote MT unit for security during
external faults.
(d) The MT function must be set with a reach that
is long enough to enable it to see all multiphase faults on the protected line.
(e) The MB function must not operate for any internal
fault for which the associated MT function does
not operate otherwise tripping will be blocked.
Summation of Combined Phase & Directional
Comparison Considerations
7. Both schemes will operate for three phase
faults that produce currents well below full
load values.
1. Both the zero and the negative sequence
schemes require the use of directional
distance measuring functions. Thus, both
schemes require potential supplies, and both
schemes are sensitive to system swings and
out-of-step conditions.
8. These schemes are a half way point between
pure phase comparison and pure directional
comparison.
As
such
they
require
coordination between the two modes of
operation as noted in item (4) above. This
problem is not present in the pure schemes.
2. The scheme using zero sequence excitation
requires mho functions that must operate for
all multi-phase faults with the possible
exception of double-phase-to-ground faults.
BLOCKING vs. TRIPPING SCHEMES
Earlier discussion in conjunction with figure 2
provides a basis for further consideration of
blocking vs tripping pilot schemes. Figure 2C
illustrates the comparer-integrator logic for a
tripping scheme using an ON-OFF type of pilot
channel. In order to trip, a receiver output is
required to be present during the half cycle that
the local current is positive. Figure 2D is
representative of a blocking pilot scheme where
tripping will take place if there is no receiver
output during the half cycle that the local
current is positive.
3. The scheme using negative sequence
excitation requires mho functions that
respond to three phase faults only.
4. Both schemes require that the MB functions
do not operate for any internal faults for
which the associated MT functions do not
operate. Since phase distance mho relays can
respond to single-phase-to-ground faults and
faults on adjacent phases, the selection of the
type of mho functions employed for any given
application requires consideration. The
factors involved in this consideration are
outside the scope of this discussion.
If we consider that an input to, or an output
from a logic box is a positive going signal, the
logic illustrated in Figures 2B and 2C assume
that a received signal at the input of a receiver
will produce a positive going voltage signal at
the output of the receiver to the relay logic. This
5. Neither scheme will misoperate as a result of
zero sequence mutual coupling between the
protected line and other parallel lines.
20
1. ON-OFF
is not always true. Some types of receivers will
produce negative (or reference) voltage outputs
when a signal is present at the input, and a
positive signal output when nothing is received.
If this were the situation in Figure 2, Figure 2C
would then represent a blocking scheme while
Figure 2D would be a tripping scheme. In some
applications where receiver outputs are
inverted, the interface between the receiver and
the relay logic includes an invertor (INV) which
in effect inverts the receiver output signal so
that a received signal produces a positive going
signal at the output of the invertor. The same
general statements regarding signal polarities
applies to the keying requirements for
transmitters. Some transmitters may require a
positive signal while others a reference or
negative signal to key them off of their
quiescent states.
2. Frequency-shift
The ON-OFF type, as the name implies, operates
with the transmitter either being keyed on or off
by the relay logic. That is, the transmitter at any
given instant is either sending an unmodulated
signal or it is sending nothing.
There are two kinds of frequency-shift
equipments. The most prevalent is the twofrequency kind. With this type, the transmitter
can send either of two closely spaced
frequencies. When no keying signal is applied to
the transmitter it sends one of these two
frequencies. When the transmitter is keyed, it
shifts to the other frequency. It is always
sending one or the other. The frequency-shift
receiver has two separate outputs one for each
of the two transmitted signal frequencies. Thus,
if the transmitter is sending the MARK
frequency the MARK output is present in the
receiver. If the transmitter is sending the SPACE
frequency, the receiver SPACE output is present.
These types of receivers are basically FM
receivers and utilize discriminators. Because of
this the SPACE and MARK outputs from the
receiver cannot both be present simultaneously.
Also, broad band noise at the input to the
receiver tends to provide a balanced signal to
the discriminator which forces its output toward
zero. If the noise is severe enough to swamp out
the real signal it can cause random receiver
output or all output to disappear.
The main point to be gained from the foregoing
discussion is that it is not always possible to
determine from a logic diagram whether a
scheme is of the blocking or tripping type unless
an indication is given as to the receiver output
voltages. This applies to frequency shift as well
as ON-OFF communication equipment.
It will become apparent from subsequent
discussion that it is extremely difficult, if not
impossible, to provide a concise rigorous
definition of the terms Blocking Scheme and
Tripping Scheme. Possibly it would be well to
proceed with a discussion of the different kinds
of channels, their characteristics, and their
application before attempting a definition.
The other kind of frequency-shift equipment is a
three-frequency type. When this type of
transmitter is in its quiescent state it sends the
center frequency. It has two separate keying
inputs so that it can be keyed to shift high or
low (MARK or SPACE) from the center
frequency. The three-frequency receiver receives
all three frequencies but provides only two
outputs to the relay logic, the high shift and low
Type of Channels
The total channel is composed of the
communication equipment itself plus the path
or link over which the signal is sent. For relaying
purposes there are two basic types of
communication equipment.
21
shift outputs. When the receiver receives the
center frequency neither the high nor low
outputs are present. Here again the MARK and
SPACE outputs (high and low) cannot both be
present simultaneously, and severe broad band
noise at the receiver inputs can result in receiver
output.
5. There is the tail off time in the receiver from
the instant the input changes until the output
changes accordingly. This time plus the tail
off time of the transmitter is called the
channel release time.
6. In ON-OFF channels the operating and release
times are not generally the same. They can
vary with frequency and attenuation.
There
are
several
characteristics
of
communication equipment directly related to
phase comparison relaying performance that
might well be discussed. Phase comparison
types of schemes compare the phase angle of a
current derived at one end of a line with a
communication signal received from the remote
end. The communication signal arrives in a
MARK-SPACE arrangement that should
represent the positive and negative half cycles
of current at the transmitted end of the line.
Actually this is not possible for several reasons.
7. In frequency-shift channels the discriminator
employed in the receiver can be balanced so
that build up and tail off times are equal, or it
can be unbalanced (biased) to the MARK or
SPACE side. For example, if it is biased
toward MARK and the input signal is
symmetrical (half cycle MARK and half cycle
SPACE), the output will be more than a half
cycle MARK and less than a half cycle SPACE.
1. There is a time lag from the instant a
transmitter is keyed until the output reflects a
change. This build up is generally a very short
time and is usually insignificant.
8. In general wide band channels tend to
operate and release faster than narrow band
channels. That is, faster channels use more
spectrum than slower channels.
2. There is the propagation time from the instant
the transmitter sends until this signal arrives
at the remote location, approximately 1 millisecond for every 180 miles of distance. The
same applies from the instant the transmitter
stops until the remote signal is gone.
It is obvious from the foregoing that the
received signal at any given terminal is not an
exact analog of the remote current. There are
techniques used in phase comparison schemes
to compensate for this and they will be
discussed subsequently. Until then it should be
assumed that the received signal provides a true
representation of the phase position of the
remote current.
3. There is the build up time in the receiver from
the instant the signal appears at its input
until the output reflects the change of state.
This time plus the build up time in the
transmitter is called the channel operating
time.
Types of Communication Links
The communication link over which the
transmitted signal is propagated to the remote
receiver can take several forms. These are noted
below.
4. There is the tail off time in the transmitter
from the instant the keying is removed until
the output signal changes or disappears. This
is generally very short and is usually
insignificant.
1. Directly over the power line (Power line
carrier)
22
Power Line Carrier Links
2. Multiplexed over the power line (Single Side
Band Carrier)
It is obvious that the performance of any
channel that utilizes the protected power line
itself as a link will be affected in some way by
faults on the power line. A fault on a
transmission line can attenuate or completely
block a signal, transmitted at one end of the
line, from being received at the remote end.
Faults external to the protected line have no
effect on the signal attenuation since
transmission lines that incorporate power line
carrier channels are trapped at each end (See
Figure 11).
3. Multiplexed over microwave (Microwave)
4. Pair of Wires (Pilot Wire)
5. Leased Facilities (Telephone Company)
(a) Metallic pilot wire
(b) Microwave
(c) Cable
A distinction is made between leased facilities
and the other (power company owned) facilities
because in many cases the telephone company
defines the characteristics of the channel
without defining the type of link.
In the case of ON-OFF power line carrier
channels, the operating frequencies of the
equipment at all terminals of the protected line
are generally the same. Thus, a signal
transmitted from any terminal is received at all
terminals. This is not a necessary requirement
for using this kind of equipment. Rather it is
desirable because the protection schemes that
use ON-OFF channels can accommodate a
single frequency arrangement and this
conserves the carrier spectrum.
The ON-OFF type of communication equipment
is used exclusively over power line carrier links.
The transmitted signal is propagated along the
power line between the transmitter and the
remote receiver. These equipments usually
operate in the frequency range of 30-200 kHz.
The frequency-shift equipments are available in
several frequency ranges. First there are those in
the audio range. These are generally employed
over single side-band, microwave, pilot wires,
and leased facilities. There are also frequency
shift channels in the power line carrier frequency
range. These are employed directly over the
power line as are the ON-OFF types of
equipment. Finally there are the frequency shift
equipments that operate in and occasionally
outside the power line carrier spectrum. These
are employed over microwave and leased
facilities.
When frequency-shift equipment is used over
power line carrier, the frequencies of each
transmitter on the line must be different from all
the others on the same line. For example, if the
communication equipment in Figure 11 is of the
frequency-shift type, the transmitter at the left
end must operate at the same frequencies as
the receiver at the right end. Also, the right end
transmitter and left end receiver must operate
at the same frequencies while the frequencies of
the two transmitters must be different. This is
necessary because with frequency-shift
equipment the transmitters associated with a
given line protection scheme are not all
generally sending the MARK or the SPACE
frequencies at the same time. Thus, if a receiver
were able to receive more than one transmitter,
it could be simultaneously receiving a MARK
23
24
The FDL and NOT1 functions provide a means
for tripping when one end of the line is open as
when picking up a faulted line from one end. For
such a condition, the SQ AMP at the open end
receives no current and so produces no output
to key its transmitter. Without a received signal
the closed end of the line cannot trip under any
conditions even in the presence of a fault. The
FDL function acts as a current detector. It is set
with a very very low pick up so that any
significant output from the mixing network
causes it to produce a continuous output. When
the mixing network outputs goes to zero, FDL
drops out causing an output from NOT1 which
in turn keys the transmitter on continuously.
This is received at the remote end to provide a
continuous signal at the bottom input to AND1.
Any fault that picks up FDH will then be tripped
at the closed end of the line.
signal from one and a SPACE signal from
another. This would not result in a workable
protection scheme.
When power line carrier channels are used,
significant losses are present in the coupling
equipment and the line itself. Depending on these
losses and the ambient noise on the line, the
transmitter power required may vary from about 1
Watt to 10 Watts and even more in extreme cases.
Consider an ON-OFF tripping type of scheme as
defined by Figure 12. For a moment assume that
FDL and NOT1 do not exist in the logic. During
an internal fault, the currents out of the mixing
(or sequence) networks at both ends of the line
are in phase with each other so that the outputs
of the SQ AMP are in phase at both ends of the
line. The transmitters at both ends of the line
are keyed on during the same half cycles that
their associated SQ AMP's are attempting to
trip via AND1. Thus, the receivers will be
supplying the bottom input to AND1, and
tripping will take place when FDH operates to
provide the third input.
If the mixing network includes a positive
sequence output, load current will keep FDL
picked up continuously. If the mixing network
includes only zero and/or negative sequence
outputs, load current will not keep FDL picked
up. Thus, with zero or negative sequence phase
comparison the receivers at both ends of the
line will be producing outputs to AND1
continuously. When a fault occurs, FDL picks-up
very fast to restore the keying function to SQ
AMP. This operation resembles a blocking
scheme, although it is often called a permissive
tripping scheme.
For external faults, the currents out of the
mixing networks at the two ends of the line will
be 180 degrees out of phase with each other.
Therefore, during the half cycle that the SQ AMP
at one end of the line is producing an output, the
one at the remote end is not, so no tripping will
take place. It should be noted that a tripping
type of scheme over an ON-OFF channel
requires transmitters of different frequency
at each end of the line so that no receiver
can receive the locally-transmitted signals;
otherwise tripping would occur during external
faults. For this reason, such schemes are not
generally applied.
Another scheme to facilitate tripping on single
end feed uses a circuit breaker 52/b switch
rather than FDL and NOT1. When the breaker is
open, the 52/b switch closes and keys the
associated transmitter on continuously. When
the breaker is closed, the 52/b switch is open
and keying is under control of the SQ AMP.
While on the surface the use of 52/b appears
simple and direct, the following problems arise
that can require more complex logic and station
wiring:
It appears that the tripping scheme as described
above has no need for an FDL function since no
blocking coordination is required as is in a
blocking scheme. However, this is not the case.
25
because absence of a signal is required in order
to trip. During external faults it is important that
the blocking signal be isolated from the fault
because loss of the signal can result in a false
trip. The line traps provide this isolation.
1. The 52/b contacts do not generally operate in
synchronism with the main poles of the
breaker so some timing functions must be
included with the logic to compensate for this.
2. In multi-breaker schemes, such as ring buses,
two breakers at each terminal are associated
with each line so 52/b switches from each
breaker are required in series.
Figures 4 and 14 illustrate phase comparison
blocking schemes with ON-OFF and frequencyshift channels respectively. Figure 4 was
discussed earlier and Figure 14 is exactly the
same except for the high frequency shift which
is not used in the protection scheme. While only
one of the two frequencies of the frequencyshift equipment is used in the protection
scheme, the second frequency does perform a
useful function. It provides a means for
continuous monitoring of the channel. Since
one of the two frequencies is always being
transmitted, it is possible to monitor the signal
at each receiver continuously and incapacitate
the protective scheme and/or provide indication
at that terminal if the signal is lost.
3. In multi-breaker schemes one of the two
breakers may be out of service but in the
closed position. This would require a bypass
of its 52/b switch which is open.
Regardless of which tripping scheme is used, it
is obvious from Figure 11 that in order to trip
either circuit breaker A or B for an internal fault
at P it is necessary to get a carrier signal through
the fault. If the fault attenuates the signal so
that this does not happen, no tripping can take
place. The amount of attenuation in signal that
is produced by the fault will depend on the type
of coupling (single phase, interphase, etc.), the
type of fault, the phase involved, and the
location of the fault on the line. The evaluation
of these factors is outside the scope of this
discussion.
Most schemes that use an ON-OFF channel are
arranged so that no transmission takes place
during normal conditions (no fault). This does
not lend itself to continuous monitoring.
However, schemes are available that periodically
start transmission of a signal at one end of a line
which, when received at the remote end,
initiates a return transmitted signal. Such
schemes can be started manually or
automatically on a time schedule. They are
called carrier check-back schemes. They can be
arranged so as not to affect the normal
operation of the scheme even in the event of a
fault during a check-back operation.
Figure 13 illustrates the same tripping scheme
as Figure 12 except that it utilizes a frequency
shift rather than an ON-OFF communication set.
The same comments apply to this scheme as do
to that of Figure 12.
A tripping scheme that operates over a power
line carrier channel runs the risk of a failure to
trip on internal faults because of signal
attenuation. During external faults the line traps
isolate the signal on the protected line from the
fault. This is of no significance because
attenuation or loss of signal on external faults
cannot result in any misoperations. Conversely,
a blocking scheme is unaffected by loss or
attenuation of signal during internal faults
For the most part, phase comparison blocking
carrier schemes use ON-OFF rather than
frequency-shift channels, possibly for one or
more of the following reasons:
1. The overall speed of the protective scheme is
directly related to the speed of the channel.
26
27
Until recently high speed frequency shift
carrier channels were not available. Even
today the ON-OFF channel is somewhat faster
than the fastest frequency-shift channel.
13 or the circuit breaker auxiliary 52/b switch could
be used.
In general, unblocking utilizes frequency-shift
channels because this permits monitoring of the
continuous blocking signals. As they are usually
applied, ON-OFF channels do not lend themselves
to monitoring because the single frequency
system transmits the same frequency from all
transmitters and the loss of any one transmitter
could not be detected. If applied in a normal
duplex frequency basis (one in each direction) the
ON-OFF channel would provide the monitoring
features at the cost of carrier spectrum. However,
this disadvantage can be overcome by the use of a
new application of ON-OFF equipment where the
transmitters at the different terminals are
operating at frequencies offset from each other yet
close enough to be nominally a single frequency
system. This application permits monitoring, and
at the same time has the advantage of a higher
channel speed than the frequency-shift channels,
while utilizing less channel spectrum in three
terminal line applications.
2. Noise at the input of an ON-OFF channel
receiver would tend to produce a blocking
signal output. Noise at the input of a
frequency-shift channel tends to drive its
output to zero which is a tripping condition (in
a blocking scheme). This tends to make the
frequency-shift blocking scheme less secure
against false tripping during external faults. It
is possible to build channel condition
detectors (signal to noise, loss of channel,
etc.) into frequency-shift channels and block
tripping when these detectors indicate
trouble, but these features increase the
complexity and the cost. This approach tends
to make the blocking scheme resemble the
tripping scheme since the receiver must now
indicate an intact channel in order to trip.
3. Aside from the ability to accommodate
continuous monitoring, the frequency-shift
channel provides little advantage over the
ON-OFF carrier channel.
Single Side Band Carrier Links
Like the carrier channels discussed above, the
single side band communication equipment is
coupled directly to the power line through
coupling capacitors and utilizes the power line
to propagate the signal. In general, the output
frequency of the single side band equipment is
in the 30-200KC carrier spectrum, while the
multiplexed channels are of the frequency-shift
type that operate in the audio range. The only
advantage of this type of arrangement is that it
provides a multi-channel equipment over a
minimum of channel spectrum with a cost
economy over single channels.
There are very few if any phase comparison
tripping schemes in service over carrier channels
mainly because of the fear that it will not always
be possible to get a trip signal through a fault.
There is another type of scheme that has recently
been gaining some favor. This is called Unblocking.
It is a cross between blocking and tripping in that
it operates in the blocking mode but the blocking
signal is sent continuously even in the quiescent
state (no fault), and so it must be turned off in
order to trip. Thus this scheme, as in the tripping
schemes previously described, must include some
means to stop the blocking signal from being
transmitted at an open terminal in order to permit
tripping of the closed remote terminal in the event
of a fault. Here again the FDL logic of Figure 12 and
Single side band carrier links have not been
used extensively in this country for protective
relaying. When they are used for this purpose
they usually include a relatively high speed
28
frequency-shift channel for the relaying function
plus a limited number of narrow band
equipments for control functions. On some
occasions a high speed ON-OFF channel has
been multiplexed over single side band. These
high speed channels usually operate in the
kilohertz range, and because of the speed
requirement are of the wide band variety.
Phase comparison relaying has been employed
over single side band links. In general, the high
speed frequency-shift type of channel is used in
a tripping scheme, and the ON-OFF type in a
blocking scheme. The same basic comments
apply to these schemes as were made above for
similar carrier schemes applied without the
single side band. The single side band
equipment adds nothing to the overall
performance of the protective relay scheme
except that it contributes to better spectrum
utilization and is economically advantageous to
use when there are several voice and control
functions in addition to relaying required
between the same terminal points.
The final R.F. amplifier of the single side band
transmitter has, by design, a certain maximum
power capability. Each of the signals that is
multiplexed over this equipment uses a fraction
of that total power depending on the percent
modulation as indicated by the following
equation.
Pn = Pt
Microwave Links
M 2
100
( )
Microwave links are quite commonly used for
protective relaying including phase comparison
schemes. However, because of the high cost of
the microwave equipment, the applications are
generally limited to cases where a large number
of control and/or monitoring functions are
needed between the same terminals as the
relaying.
where:
Pt = total capability of the output amplifier
Pn = Power output of a given multiplexed
signal
M = Percent modulation by the given
multiplexed signal
Since microwave links propagate through the
atmosphere, rather than over the power line,
they are generally unaffected by faults and noise
on the power system. Thus, with a microwave
link there is no problem of getting a signal
through the fault, so tripping type schemes are
very acceptable. On the other hand, since there
is a possibility of fading of the microwave signal,
there is some reluctance to use it in blocking
schemes for fear of false tripping in the event of
a fade during a nearby external fault. However,
blocking schemes are used occasionally mainly
because the tripping scheme requires special
circuitry (as described earlier) in order to trip on
single-end feed to a fault.
It should be noted that the total percent
modulation for all multiplexed signals must not
exceed 100 percent. Thus, if only two signals are
multiplexed, each at 50 percent modulation,
each signal will have one fourth the total power.
For four signals equally muitiplexed only one
sixteenth the total power will be available to
each. When relaying is used in conjunction with
voice and control functions on single side band
it is often necessary to exhalt the relaying
channel signal when it is called on to block or
trip. This results in the particular signal
temporarily usurping most, if not all, of the total
available power at the expense of the other
functions.
The communication equipment multiplexed on
to a microwave system for protective relaying is
29
tripping or blocking mode as indicated in
Figures 13 and 14 respectively. Aside from the
considerations involved in tripping for a fault
with single-end feed, which were discussed
previously, the selection between a blocking and
a tripping scheme will generally result from a
compromise between security and reliability. In
order to make such a selection, consideration of
the pilot pair, its protection, and its physical
location in relation to power conductors must
be evaluated.
invariably of the frequency-shift type, and
usually of the high speed variety. Figures 13 and
14 are representative of the tripping and
blocking schemes respectively. Since, as
mentioned above, the microwave signal can
fade, some of the frequency-shift receiver
equipments include channel status detectors
that operate into the relay logic to incapacitate
all tripping when the channel conditions are not
normal. The ability to trip is then automatically
reinstated when normality returns. With such an
arrangement, complete loss of receiver output
would incapacitate tripping. If the scheme were
a blocking scheme similar to that of Figure 14,
complete loss of channel during an external
fault would permit a false trip unless an
incapacitating feature were included in the
scheme.
In general, a high speed channel would require
pilot wires that have a frequency response that
is somewhat better than the standard telephone
voice circuits.
Possibly because of the uncertainties of channel
characteristics, plus the availability of pilot wire
relays that are much lower in overall cost, phase
comparison over privately owned pilot wires is
not a common application.
The receiver has only two outputs (high and
low). Since the scheme trips on internal faults
during the absence of the low-shift output, and
since the absence of both the low and high shift
outputs incapacitates tripping (where used), the
implied requirement for tripping is the presence
of the high-shift receiver output. While such a
scheme is called a blocking scheme it appears to
be, at least by implication, a tripping scheme.
Leased (Telephone Company) Facilities
In any case, there is nothing about a microwave
channel to alter the previous discussion
concerning phase comparison protection. The
same basic schemes may be used with the
understanding that the microwave signal can
fade on occasion. For the most part, phase
comparison relaying schemes over microwave
channels have been of the tripping types.
There has been some use of phase comparison
relaying over leased facilities including voice
grade pilot wire circuits. In general, if a
customer
requires
or
specifies
the
characteristics of a leased channel, the local
telephone company could provide this link over
microwave, cable, even pilot wires, or a
combination of these. In such cases the
selection between tripping and blocking
schemes will depend on the performance of the
channel as specified. The same basic schemes of
Figures 13 and 14 would apply.
Pilot Wire Links
Summation of Blocking vs. Tripping Schemes
There are very few, if any, privately owned pilot
wires in this country that are used as a link in
phase comparison schemes. However, such
applications would require a frequency-shift
communication equipment used in either a
The foregoing discussion of blocking and
tripping schemes was presented without the
benefit of a concise definition of these terms. As
indicated in the discussion, the difficulty of
making such definitions which would always
30
maximum time for the single phasecomparison. The minimum times for both
schemes will be the same. This difference in
maximum time results because a fault could
occur at such an instant in time when the
current is just going negative. Under such
conditions, the single phase-comparison would
have to wait till the next positive half cycle
while the dual phase-comparison could trip on
the upcoming negative half cycle.
apply is brought about by the channel status
feature used in some frequency-shift blocking
schemes. Such arrangements tend to be
hybrids. Thus, the following simple definitions
exclude any considerations of channel status
features:
•
A blocking scheme is one that requires a
specific output signal from the associated
receiver in order to block tripping. Tripping
can only take place during the time that this
signal is absent.
•
A tripping scheme is one that requires a
specific output signal from the associated
receiver in order to permit tripping. Tripping
can only take place during the time that this
signal is present.
•
Where channel status logic is used, these
definitions will have to be modified to meet
the exact logic of the scheme.
While, as a general rule, high speed operation
and security are on opposite sides of the coin, it
is possible to design dual phase-comparison
schemes that can provide the added speed with
little or no loss in security. However, these
schemes are somewhat more complex than
equivalent single phase-comparison schemes.
Figure 15 illustrates the dual phase-comparison
tripping scheme that is the counterpart of the
single phase-comparison scheme of Figure 13.
The differences are noted below:
1. The dual scheme uses two separate
comparer-integrator combinations, one for
the positive half cycle and the other for the
negative half cycle.
In general, the selection of a blocking or a
tripping scheme is one that should be made in
conjunction with the chosen channel and with a
knowledge of the channel characteristics in the
face of system noise. Many different
combinations are possible, but of these, only a
selected few will meet any given set of
requirements.
2. A three-frequency, frequency-shift channel is
used in dual phase comparison. The high-shift
operates in conjunction with the positive half
cycle while the low-shift works with the
negative half cycle. When the channel is not
keyed to either high or low, it operates on the
center frequency. There is no center frequency
output from the receiver into the relay
tripping logic.
SINGLE vs. DUAL PHASE-COMPARISON
In all the phase-comparison schemes described
so far, a trip attempt is made only every other
half cycle. In the examples illustrated, this was
every positive half cycle. Such schemes are
termed single phase-comparison as against dual
phase-comparison where a trip attempt is made
every half cycle, positive and negative.
3. AND3 is included to make it impossible to key
both frequencies simultaneously. It also gives
preference to the low-shift which is sent
continuously when FDL is dropped out. Thus,
on single-end feed tripping can take place
only on the negative half cycle.
The only advantage of dual-comparison is that
its maximum operating time to trip on internal
faults will be a half cycle faster than the
31
blocking and in the tripping modes. The overall
performance of the scheme will be dependent
on the characteristics of the channel selected.
While dual phase-comparison will reduce the
maximum tripping time, it does so at the
expense of simplicity and possibly some
security depending on how it is accomplished.
The center frequency, while not actually used in
the relay tripping logic, adds security to the
scheme during transient conditions.
The dual phase-comparison scheme of Figure 15
could be modified to operate over a twofrequency,
frequency-shift
channel
by
eliminating AND3, FDL, NOT1, and the center
frequency. The transmitter could then be
arranged to send the low-shift frequency
continuously except when keyed by the positive
SQ. AMP to the high shift frequency. This
arrangement, though simpler than the threefrequency scheme, is deemed to be less secure.
REFINEMENTS TO BASIC SCHEMES
There are a number of standard refinements
that are required and normally included in all
phase comparison schemes. These will be
discussed in terms of the basic blocking scheme
of Figure 4, but will apply generally to all
schemes, sometimes in a somewhat different
form.
Figure 16 illustrates a dual phase-comparison
blocking scheme using a two-frequency,
frequency-shift channel. Since one or the other
of the two frequencies must be on at all times,
and since both are blocking frequencies, there
appears to be little need for an FDL function.
Thus, it is not included. When the transmitter is
not keyed, it sends low-shift continuously and
when it is keyed by the negative squaring
amplifier, it shifts to high for the negative half
cycle. This scheme is simpler than that of Figure
15 but probably is not as secure.
SYMMETRY ADJUSTMENT
As was noted in a previous section, receivers are
not always symmetrical in their response. That
is, if a transmitter is keyed on and off
symmetrically every half cycle, the remote
receiver output would not necessarily
correspond exactly to the keying signal. For
example, if an ON-OFF transmitter were keyed
on for a half cycle and then off for a half cycle,
and so on, the remote receiver output might be
on for more than a half cycle and off for less
than a half cycle. This effect is primarily due to
the filter response in the receiver and is
common with ON-OFF type of equipment. It is
not a constant value but rather depends on
operating frequencies as well as received signal
strength. Thus, this asymmetry may vary from
equipment to equipment and from time to time
(as atmospheric conditions change) in service.
There does not appear to be any good purpose
for a three-frequency channel in dual phasecomparison blocking schemes since the center
frequency would not add to the security, or
otherwise improve the performance.
It is interesting to note that a dual phasecomparison scheme using an ON-OFF channel
would have to be a combined blocking and
tripping scheme. During one polarity of half
cycle, it would have to trip on absence of any
received signal (blocking), and on the other
polarity of half cycle, it would have to trip in the
presence of the received signal.
Frequency shift channels are generally
symmetrical in their response when the
discriminator in the receiver is balanced. If the
discriminator is biased to one side or the other
the receiver output tends to favor the side to
which it is biased.
In general, it may be concluded that dual phasecomparison may be accomplished in the
32
33
this phase delay but there is a way to
compensate for it. This compensation is
accomplished by the phase delay timer (0-8/0-8)
in the comparer input circuit.
Because of this, all phase comparison schemes
that may operate with asymmetrical channels
are equipped with a symmetry adjustment.
Figure 17 illustrates the same scheme as that of
Figure 4 except that it includes a symmetry
adjustment and a phase delay adjustment
feature. The phase delay adjustment will be
discussed subsequently.
PHASE DELAY ADJUSTMENT
The phase delay timer (0-8/0-8) is a timer that is
set with a pickup and a dropout delay that are
equal to each other so that it introduces a phase
delay. Its output is the same shape as that of the
squaring amplifier but delayed in time by the
setting. This time delay setting is made in the
field to be just equal to the sum of the three
delays (symmetry adjustment, propagation, and
receiver) discussed above. Thus, with this
arrangement in the blocking scheme of Figure
17, an external fault would produce a receiver
output exactly in phase and symmetrical with
the output of the phase delay timer. This is
necessary for proper blocking. For internal faults
the output from the phase delay timer would be
symmetrical with, but 180 degrees out of phase
with the receiver output. This is necessary for
tripping. It should be recognized that any errors
in these adjustments can reduce the tripping
margins for internal faults and/or reduce the
blocking margins during external faults.
The symmetry adjustment (0-3/0-3) is in the
keying circuit to the transmitter. It is set with
either a time delay pickup or a time delay drop
out depending on whether the receiver
elongates or shortens the received signal. The
time setting is made in the field after the
transmitters, receivers, and coupling equipment
have all been tuned and adjusted for proper
sensitivity. The proper setting is obtained by
keying the transmitter on and off by means of a
symmetrical sinusoidal output from the mixing
network. Then, while this is taking place, the
time delay pickup or dropout of 0-3/0-3 is
adjusted so that the remote receiver yields a
symmetrical output.
The results of the symmetry adjustment are
twofold. First, the remote receiver output is
symmetrical, and second the keying signal may
be phase shifted in the lagging direction from
the actual squaring amplifier output. This latter
result is not desirable, but fortunately it may be
mitigated. It is obvious that if the keying signal
is shifted in phase from the local squaring
amplifier output, the remote received signal will
be phase shifted from the current at the keying
terminal. In addition to this there is the
propagation delay in getting the communication
signal from the transmitter to the remote
receiver (1 millisecond per 186 miles) plus the
delay in the receiver itself. All of these add to
each other to produce a receiver output that
may be significantly phase delayed from the
current at the remote end of the line. This is
undesirable because it introduces an error in the
phase comparison. There is no way to eliminate
It is interesting to note that the setting of the
phase delay timer is dependent on the channel
operating time, and that the total tripping time of
the scheme is affected by this timer setting.
Thus, the tripping speed of the scheme is to that
degree dependent on the channel operating time.
TRANSIENT BLOCKING
Transient blocking is a feature that is included in
all phase comparison schemes. It adds to the
security of the scheme during and immediately
after the clearing of external faults. Figure 18 is
a representation of Figure 17 except with the
transient blocking logic added. This consists of
AND6 and the 20-40/40 timer.
34
The basic logic of the transient blocking scheme is
such that if a fault is detected, as indicated by the
operation of FDH, but no trip takes place, as
indicated by no output from the integrator (3/18)
timer, AND/6 produces an output to the transient
blocking timer (20-40/40). If this condition persists
for a time that is long enough for the transient
blocking timer to produce an output, tripping is
blocked via the NOT input to AND5. This blocking
of a trip output persists for 40 milliseconds after
AND6 output disappears as a result of FDH
resetting or 3/18 producing an output.
MULTI-TERMINAL LINES
Up to this point these discussions have
pertained principally to two-terminal lines.
Phase comparison schemes are often applied to
lines having more than two terminals and those
applications differ somewhat depending on the
channel equipment.
ON-OFF CHANNEL
The ON-OFF channel equipment is invariably
used in blocking type carrier schemes similar to
that of Figure 18. Since this type of scheme
utilizes only one common frequency for all the
transmitters and receivers, Figure 18 will apply
to multi-terminal lines as well as two terminal
lines. A blocking signal sent from any terminal
will be received at all the other terminals to
provide the necessary blocking via the single
receiver at that terminal.
The pickup time delay setting of the transient
blocking timer is somewhat longer than the
expected time difference between FDH pickup
and a 3/18 output during an internal fault. This
insures no delay in tripping in the event of an
internal fault, as well as prolonged blocking
during the clearing of an external fault during
which transient power reversals may tend to
cause false tripping. Actually this transient
blocking feature provides an added margin of
security since the scheme is designed to be
secure without this circuitry.
Since all the receivers and transmitters operate
at the same frequency, each receiver will receive
its own local transmitters as well as all the
remote transmitters. It will be recalled that the
symmetry adjustment in the keying circuit is
based on the response of the remote receiver.
Since the attenuation between a transmitter
and its local receiver may be quite different from
what it is between the same transmitters and a
remote receiver, it is reasonable to expect that
the output of a receiver responding to a local
transmitter will not be symmetrical. Thus,
during internal faults, even assuming that the
currents at all the line terminals are exactly in
phase, it is quite possible that the receivers will
produce outputs somewhat longer than a half
cycle. This is true for two-terminal lines as well
as multi-terminal lines but it can be more
exaggerated on multi-terminal lines because of
the added transmitters and adjustments that
may not be exactly correct.
SUMMATION
It should be recognized that while the
discussions in this section were strongly
oriented toward a blocking scheme over an ONOFF channel, the same or similar comments and
circuits apply to all the other schemes. In some
cases, for one reason or another, these functions
may differ somewhat in arrangement from the
logic illustrated in Figure 18. For example, it is
obvious that the symmetry adjustment may be
left out of the keying circuit if it is included in
the output of the receiver, and in some
applications this is done. In some cases of
frequency-shift equipment, the symmetry
adjustment is an integral part of the receiver,
and so would not be shown on the logic
diagram. In any case these functions are
included in all schemes in one form or another.
35
36
While the situation described above will reduce
the margin for tripping on internal faults, it is
not generally a problem if proper sensitivity
settings are made on the receivers, and careful
and proper symmetry and phase delay
adjustments are made on the relays as well. The
use of hybrids to attenuate the strength of the
transmitted signal as received by the local
receiver can also be helpful. These are available
on some of the newer carrier sets and they tend
to make the received local transmitter signals
appear weaker than the remote transmitter
signals.
STANDARD SCHEMES
In the foregoing discussions it was pointed out
that phase comparison relaying can be
accomplished in many different ways. These
differences relate to such factors as:
1. The type of excitation that is used.
2. The type of transmitter-receiver that is used.
3. The type of channel link that is used.
4. The operating mode – blocking vs. tripping,
single vs. dual, mho supervision vs. no mho
supervision.
It is of interest to recall in passing that in mixed
excitation (positive plus negative) schemes
during internal faults, the mixing network
output currents at all terminals are not exactly in
phase with each other. This in itself cuts down
the tripping margin, and any additional loss of
margin from the items discussed above is that
much more undesirable.
While all the different combinations were
discussed and illustrated to some degree, not all
of them are used in practice. For the most part
there are some so-called standard schemes that
are most popular because of a combination of
such factors as overall performance, cost, and
simplicity. It should also be recognized that the
discussions and logic diagrams discussed were
basic and generally rudimentary in order to
present the fundamental concepts. Actually the
various schemes often involve more detailed
considerations and may appear somewhat
different when their logic diagrams are
compared to those discussed. However, the
basic concepts are the same.
FREQUENCY-SHIFT CHANNEL
Frequency-shift channels are generally used in
tripping type schemes. Figure 19 illustrates a
three-terminal line tripping scheme using a
frequency-shift channel. This arrangement
requires two receivers at each terminal. One
receiver is required for each remote transmitter
because each transmitter is operated at a
different frequency. In order to trip, a high-shift
output is required from both receivers
concurrently to AND2. A two-terminal line
scheme would require only one receiver which
would operate directly into AND1 without the
need for AND2.
Table II presents a listing of typical schemes in
use today. It will be noted that all the schemes
listed in this table except the last two provide
complete protection against single-phase-toground as well as multi-phase faults. The last
scheme requires additional equipment for threephase faults. The next to last scheme requires
additional equipment for all multi-phase faults.
Frequency-shift channels are inherently
symmetrical, and they do not receive their own
locally-transmitted signals. Because of this,
schemes using this type of channel are not
generally troubled by the symmetry problems of
the ON-OFF carrier channels.
37
38
Table II. Popular Phase Comparison Schemes
39